1. Technical Field
The present invention relates to a display device having an electrophoretic display panel containing electrophoretic particles, and to a timepiece having the display device.
2. Related Art
Display devices having a display unit for displaying the time and a control means for controlling displaying content on the display unit are known from the literature. An example of such a display device is the display device (electrophoretic display panel) having electrophoretic particles as taught in Japanese Unexamined Patent Appl. Pub. JP-A-S52-70791.
The electrophoretic display panel taught in JP-A-S52-70791 has an electrophoretic dispersion contained between a transparent common electrode and segment electrodes. The electrophoretic display dispersion is black and contains negatively charged color particles and positively charged white particles. When a potential difference is created between the transparent electrode and the segment electrodes, the charged particles of one color migrate to the transparent electrode side, the particles of the other color migrate to the segment electrode side, and the color of the particles that migrated to the transparent electrode side is visible to the viewer. By controlling the voltage applied to the common electrode and the segment electrodes and the time the voltage is applied, the migration of particles to each of the electrodes can be adjusted and the gray level of the displayed color can be adjusted. Text and other information can be displayed by using a plurality of segments containing the electrophoretic dispersion.
A problem with this electrophoretic display panel is that the process for displaying the desired gray level is difficult because it is difficult to achieve a proportional relationship between the number of positive and negative voltage pulses applied to the electrophoretic dispersion and the gray level of the displayed color.
More specifically, the gray level changes on a saturation curve in an electrophoretic display panel so that when one pulse is applied to change from a black display state to white and to change from a white display state to black, the gray level may change abruptly. On the other hand, when one pulse is applied to change from an intermediate gray level to black or to white, the time the one voltage pulse is applied varies according to the current display state (gray level). The number of applied pulses must therefore be controlled to follow the saturation curve of the displayed gray level, and the process required to display the desired gray level becomes complicated.